"Eliminating corruption and conflicts of interest among our highest elected officials and restoring the faith of the American people in our government — stated goals of the Democratic Party in this campaign and in the next Congress — demand nothing less," the groups added.

Trump is the first president in decades who hasn't released his tax returns. He has said he won't release them while under audit by the IRS, though the agency has said that nothing prevents people from releasing their own tax information. He has also said that the public doesn't care about his tax returns.

Under federal tax law, the chairmen of Congress's tax committees can request tax returns from the Treasury Department and review them in a closed session. After that, the committees could vote to make parts or all of the returns public.

Efforts by congressional Democrats to get Congress to request Trump's tax returns have been unsuccessful so far because Republicans control both chambers of Congress. But if Democrats take control of at least one chamber in the midterm elections, they would be able to request the returns from the Treasury. Both Wyden and Neal have said they would seek the returns if they become chairmen, but neither of them have committed to making all of the documents public.

The liberal groups — which include Not One Penny, Indivisible and Americans for Tax Fairness — said that the New York Times's story on Trump's tax schemes highlights "the need for transparency."

The Times's report, which Trump has called a "hit piece," detailed practices that were undertaken by Trump and his family so that the president's parents could avoid gift and estate taxes.

If Trump's returns were public, "we would ... be better able to determine whether the president has been paying the taxes he owes, or whether he has flouted the law or pushed the legal boundaries — in other words, whether he has continued the kind of tax dodging exposed by the Times," the groups wrote.

They also said that it's important for Trump's returns to become public so that the American people can see how Trump benefits from the tax-cut law he signed last year and can see if he has any foreign bank accounts or pays taxes to any foreign governments. The groups want Democrats to obtain Trump's personal tax returns as well as those of his business entities.

"Disclosure of the president’s tax returns would allow the public to better understand the Trump Organization’s finances to assess potential or actual conflicts of interest," they wrote.